1. Field of the Invention
The subject of the invention is an airfoil attachment holding an airfoil root in a broach fitting.
2. Description of the Related Art
This attachment can be used for usually metallic supports of composite airfoils, and more particularly for devices for which the airfoils have a variable pitch, in other words their inclination relative to the rotation direction varies depending on the actual speed so as to improve the aerodynamic performances and efficiency of the engine to which they belong.
The strength of the assembly between the airfoil and its attachment must be respected to prevent any risk of detachment of an airfoil during operation, with an extremely high kinetic energy that can cause corresponding damage in the adjacent parts of the aircraft. A satisfactory attachment may satisfy one of two designs; in a first design, the attachment is designed to resist all loads that are normally applied to it; and in the other design, initiating damage is accepted provided that it cannot lead to an immediate complete rupture and detachment of the airfoil, and the attachment will then be replaced during a maintenance operation. The first safe-life design has the disadvantage that its design is more expensive because it is important to guard against more numerous or more restrictive conditions. The second fail-safe design is preferred in this case, but it is also more difficult to apply appropriately in some circumstances, for example when the airfoil and its attachment are parts made of different materials made in a single piece assembly for which the strength is uncertain, because their connection depends partly on a bond.
For this invention, we will consider a different type of airfoil attachment comprising a support and two opposite flanks that rise up on this support delimiting a broach compartment that retains an airfoil root. The assembly is usually made by an airfoil root that is wider than its aerodynamic part and with flanks that overhang the root, delimiting a narrower opening of the compartment than the root. This design is described in other documents including document FR-A-2 933 955 belonging to the applicant. However, there is also a French patent application registered under number 10 51 116 in which the airfoil root is bolted into the attachment according to a fail-safe design representative of a different assembly. The main forces to which the support is exposed during operation originate from centrifugal forces applied to the airfoil that tend to pull the airfoil root out from the compartment; these forces are transmitted to the flanks in the form of tension, possibly accompanied by bending that tends to separate the flanks from each other. Cracks can appear vertically in line with the flanks on which the airfoil root bears during operation, or at a region at which the flanks are connected to the support, where stresses due to bending are highest. These cracks can propagate along the flanks and break them, which would be followed by detachment of the airfoil.